Passage V

A cathode-ray tube (CRT) is a sealed, evacuated glass tube with a filament at one end and a fluorescent screen at the other end (see Figure 1).

When heated, the filament emits cathode rays that are accelerated by an electric potential, V, toward a barrier having a pinhole. Beyond the barrier are 2 conducting plates, each of length L, that have an electric field, E, between them. (The direction of E can be upward or downward; in Figure 1, it is downward.) Any rays that pass through the pinhole travel through the field and strike the screen, producing a bright spot of visible light.

A group of students performed 3 studies on various CRTs, each of which had a ruler taped to the outer surface of the screen (see Figure 2) to measure a spot's vertical location, y (in centimeters, cm).

Study 1

The students obtained a CRT having L = 2.5 cm. They set V to 1.0 kilovolt (kV), varied both the direction and the magnitude (in newtons per coulomb, N/C) of E, and recorded the resulting values of y (see table 1).

Study 2

Using the CRT from study 1, the students set the magnitude of E to 1.0×10⁴ N/C, varied V, and recorded the resulting values of y (see Table 2).

Study 3

The students obtained various CRTs, each having a different L. For each CRT, they set V to 1.0 kV, set the magnitude of E to 1.0×10⁴ N/C, and recorded their resulting value of y (see Table 3)